Metal-air batteries and metal-air fuel cells are very promising technologies that provide alternatives to the currently predominant fossil fuels, for energy conversion. Metals such as zinc, aluminum and lithium can be used as the fuel for the metal-air batteries/fuel cells. During battery/fuel cell discharge, oxidation of these metals (i.e., Zn and Al) occurs on the anode and releases electrons that are transported via an external circuit to the cathode where oxygen reduction reaction converts the oxygen from air to hydroxide ions. Depletion of the metal fuel is inevitable in the primary metal-air battery/fuel cell, thus a continuous supply of metal is required for long term operation.
Introducing oxygen evolution on the cathode can mitigate this effect by allowing the regeneration of zinc oxide on the anode. However, the oxygen reduction and oxygen evolution reactions have large overpotentials and sluggish reaction kinetics. Therefore, to realize large scale application of metal air battery/fuel cells, active, stable and affordable catalysts are required to improve device performance.
Previous approaches to catalysts for metal air batteries are reported in numerous journal articles and patents. Jörissen et al. 1 reviewed many bifunctional catalysts made with various materials including the perovskite, spinel and pyrochlore type mixed metal oxides. However, the authors indicate that further research on this topic is needed. Lu et al. 2 describe a bifunctional catalyst based on platinum and gold, however the high cost of the catalyst discourages large scale implementation.
In US2007/0166602 A1,3 oxygen reduction catalyst and various oxides (i.e., CoWO4, La2O3) are combined to show high bifunctional activity. In US2007/0111095,4 manganese oxide contained in octahedral molecular seives was used as catalyst for metal air electrodes. In US2004/0048215,5 a metal cell containing a two layer cathode structure, used AgMnO4 as a catalyst precursor to produce a fine dispersion of MnO2 and Ag. However, the resultant cathode is not bifunctional.
Another approach involved introducing catalysts that are innately bifunctional, that is, one catalyst that has the ability to catalyze oxygen reduction and evolution reactions 6. However, such bifunctional materials were found to be limited by low activity and current densities.
Despite the various approaches described above, there remains a need for catalysts for use in metal air batteries and fuel cells having good activity and stability.